The muscle spindle, one of the major sensory receptors in vertebrate skeletal muscles plays an important role in reflex and volitional control of muscle activity, being extremely sensitive to changes in muscle length. To stabilize their sensitivity, sensory terminals of muscle spindle should be protected against mechanical as well as ionic disturbances during muscle activity. Both capsule and capsular space of muscle spindles appear to provide such protection. The overall objective of this proposal is to determine the electrical properties and active transport mechanisms of the capsule of the mammalian muscle spindle and to test how these properties influence sensory activities. I propose the following projects: 1) To determine specific resistance and capacitance of the capsule. 2) To test how acid mucopolysaccharides present in the capsular space (or periaxial space) affect ionic activities in the space measured with ion selective microelectrode. 3) To test whether the transcapsular potential is maintained by metabolic activity of capsular cells and how effectively the spindle sensory ending in protected against ionic disturbance during muscle activity. Techniques will include voltage clamp, measurement of transcapsular voltage changes in different external bathing solutions, and flux measurement of radioisotopes (on snake spindles). 4) To test the degree of protection by the capsule of the sensory endings by recording sensory discharge or receptor potential in the bathing solution with varying concentrations of K+ and pH. 5) To test the composition of mucopolysaccharides in the capsular space by electron microscopy and hyaluronidase. 6) To examine pinocytotic transport of mucopolysaccharides from outside medium into the capsular space using fluorescent dye, acridine orange bound to mucopolysaccharides and/or S35-labelled mucopolysa accharides.